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The largest American rocket currently in use blasted off from the
California coast today (Jan. 20) carrying a new U.S. spy
satellite on a classified mission, making it the biggest rocket
launch ever from the country's West Coast.

The unmanned
Delta 4-Heavy rocket lifted off from Vandenberg Air Force
Base at 1:10 p.m. PST (4:10 p.m. EST, 2100 GMT) carrying a secret
satellite for the U.S. National Reconnaissance Office.

While the exact mission the satellite – known only as NROL-49 –
is classified, the rocket's launch was unmistakable. The rocket
soared into calm afternoon skies and was clearly visible in
television cameras until the classified mission entered a media
blackout 6 1/2 minutes after launch.

Big American rocket

The Delta 4-Heavy rocket is a massive unmanned booster. It stands
235 feet (72 meters) high, making it just a bit taller than a
23-story building.

Today's launch marked the fifth flight of the Delta 4-Heavy
rocket since its debut in 2004. All four previous launches have
been from the Cape Canaveral Air Force Station in Florida.

For its West Coast debut, the Delta 4-Heavy blasted off from
Vandenberg's Space Launch Complex 6, or SLC-6. It is the largest
rocket to launch from the West Coast since the
Titan 4B rockets retired in 2005.

"This launch marks a significant milestone in our nation's space
capability," said Air Force launch director Lt. Col. Brady
Hauboldt before the liftoff. "We've really restored a national
capability for heavy lift on the Western Range, something we have
not had since the last of the Titan 4Bs flew out of Vandenberg.
This extends our ability to cost effectively deliver payloads of
all sizes and compliments."

Launch pad's shuttle legacy

The SLC-6 launching pad was originally built in 1966 to
loft the Air Force's Manned Orbiting Laboratory - a planned
military space station at the time - and was later adapted for
launching
NASA space shuttles from California. NASA did perform some
launch pad tests using its shuttle prototype Enterprise in 1985,
but no space shuttle missions ever blasted off from California.

The Air Force spent three years and $100 million to modify the
launch pad to support Delta 4-Heavy rockets, Hauboldt said.

The launch pad upgrades give the U.S. defense department the
opportunity to launch massive spy satellites like those typically
carried by Delta 4-Heavy rockets into polar orbits — the ultimate
high ground for observing the entire planet's surface, according to
Spaceflight Now.

The last launch from the SLC-6 pad was in 2006, when a regular
Delta 4 rocket sent up a U.S. military weather satellite.

"Today’s launch was the second Delta IV Heavy launch for the NRO
in two months and was also the largest rocket ever launched from
the West Coast," said Jim Sponnick, vice president of mission
operations for the United Launch Alliance. "Most important is the
critical NRO payload launched today, which will significantly
enhance the effectiveness and safety of the brave men and women
defending our nation every day."

Stacking up huge rockets

Like its name suggests, the Delta 4-Heavy rocket is a heavy-lift
version of the expendable Delta 4 rocket, which is built by the
United Launch Alliance — a joint venture of Lockheed Martin and
Boeing. The rocket is actually made up of three boosters, each
called a Common Booster Core, that are stacked side-by-side to
give the vehicle a three-column look.

Each Delta 4-Heavy rocket is capable of launching satellites or
other payloads of up to 24 tons into low-Earth orbit, and up to
11 tons into the higher geosynchronous orbits commonly used by
navigation and communications satellites.

While the Delta 4-Heavy is the largest American rocket in service
today, it is not the country's most powerful booster. The
liquid-fueled Delta 4-Heavy generates about 2 million pounds of
thrust.

The next United Launch Alliance mission to fly is scheduled for
March 4. That mission will use an unmanned Atlas 5 rocket to
launch the Orbital Test Vehicle 2, the second test flight of the
Air Force's robotic
X-37B space plane.

This story has been corrected to note that the SLC-6 launch
complex was originally built in 1966 for the Air Force's Manned
Orbiting Laboratory program and later adapted for NASA shuttles.
You can follow SPACE.com Managing Editor Tariq Malik on Twitter
@tariqjmalik.